On Thu, May 21, 2020 at 02:17:20PM -0700, Yu-cheng Yu wrote:

Introduce a quick test to verify shadow stack and IBT are working.

Cool! :)

I'd love to see either more of a commit log or more comments in the test
code itself. I had to spend a bit of time trying to understand how the
test was working. (i.e. using ucontext to "reset", using segv handler to
catch some of them, etc.) I have not yet figured out why you need to
send USR1/USR2 for two of them instead of direct calls?

More notes below...

I think you likely want three states: no signal, failed, and okay.
Perhaps -1 for "no signal" like you have above, zero for failed, and 1
for okay.

+int test_id;
+
+void stack_hacked(unsigned long x)
+{
+	result[test_id] = -1;

So this is set to 0: "I absolutely bypassed the protection".

+	test_id++;
+	setcontext(&ucp);
+}
+
+#pragma GCC push_options
+#pragma GCC optimize ("O0")

Can you avoid compiler-specific pragmas? (Or verify that Clang also
behaves correctly here?) Maybe it's better to just build the entire file
with -O0 in the Makefile?

+void ibt_violation(void)
+{
+#ifdef __i386__
+	asm volatile("lea 1f, %eax");
+	asm volatile("jmp *%eax");
+#else
+	asm volatile("lea 1f, %rax");
+	asm volatile("jmp *%rax");
+#endif
+	asm volatile("1:");
+	result[test_id] = -1;

Set to 0, and if the segv doesn't see it, we know for sure it failed.

+	test_id++;
+	setcontext(&ucp);
+}
+
+void shstk_violation(void)
+{
+#ifdef __i386__
+	unsigned long x = 0;
+
+	((unsigned long *)&x)[2] = (unsigned long)stack_hacked;
+#else
+	unsigned long long x = 0;
+
+	((unsigned long long *)&x)[2] = (unsigned long)stack_hacked;
+#endif
+}
+#pragma GCC pop_options
+
+void segv_handler(int signum, siginfo_t *si, void *uc)
+{

Does anything in siginfo_t indicate which kind of failure you're
detecting? It'd be nice to verify test_id matches the failure mode being
tested.

+	result[test_id] = 0;
+	test_id++;
+	setcontext(&ucp);
+}
+
+void user1_handler(int signum, siginfo_t *si, void *uc)
+{
+	shstk_violation();
+}
+
+void user2_handler(int signum, siginfo_t *si, void *uc)
+{
+	ibt_violation();
+}
+
+int main(int argc, char *argv[])
+{
+	struct sigaction sa;
+	int r;
+
+	r = sigemptyset(&sa.sa_mask);
+	if (r)
+		return -1;
+
+	sa.sa_flags = SA_SIGINFO;
+
+	/*
+	 * Control protection fault handler
+	 */
+	sa.sa_sigaction = segv_handler;
+	r = sigaction(SIGSEGV, &sa, NULL);
+	if (r)
+		return -1;
+
+	/*
+	 * Handler to test Shadow stack
+	 */
+	sa.sa_sigaction = user1_handler;
+	r = sigaction(SIGUSR1, &sa, NULL);
+	if (r)
+		return -1;
+
+	/*
+	 * Handler to test IBT
+	 */
+	sa.sa_sigaction = user2_handler;
+	r = sigaction(SIGUSR2, &sa, NULL);
+	if (r)
+		return -1;
+
+	test_id = 0;
+	r = getcontext(&ucp);
+	if (r)
+		return -1;
+
+	if (test_id == 0)
+		shstk_violation();
+	else if (test_id == 1)
+		ibt_violation();
+	else if (test_id == 2)
+		raise(SIGUSR1);
+	else if (test_id == 3)
+		raise(SIGUSR2);
+
+	r = 0;
+	printf("[%s]\tShadow stack\n", result[0] ? "FAIL":"OK");

Then these are result[0] == -1 ? "untested" : (result[0] ? "OK" : "FAIL"))

+	r += result[0];
+	printf("[%s]\tIBT\n", result[1] ? "FAIL":"OK");
+	r += result[1];
+	printf("[%s]\tShadow stack in signal\n", result[2] ? "FAIL":"OK");
+	r += result[2];
+	printf("[%s]\tIBT in signal\n", result[3] ? "FAIL":"OK");
+	r += result[3];
+	return r;
+}
-- 
2.21.0

-- 
Kees Cook